Coating chemistry

Coating chemistry of paints

1. Coating Chemistry

2. Properties of a Coating Desirable coating properties include: Chemical Resistance Water Resistance Ease of Application Adhesion to Substrate Cohesive Strength Flexibility and Elongation Impact Resistance Abrasion Resistance Temperature Resistance Dielectric Strength

3. Classification of Coatings Coatings are broadly classified as organic or inorganic. Organic coatings are those whose binders have been made from living or once- living things. All organic coatings contain carbon bond. Inorganic coatings use inorganic binder materials, most commonly based on either silicone or silicates. The major difference between organic and inorganic coatings is heat resistance.

4. Composition Liquid-applied coating components are characterized by the following terms: Pi g me n t Vehicle – Binder/Resin – Solvent Additi v es Resin Solvent Vehicle Pigment / Additives Coating Components

5. Pigm e nt Pigments may be used to: Impart color Provide inhibitor protection Provide a form of cathodic protection Modify mechanical or electrical properties

6. A ddit i v es Additives are most commonly liquid components of a coating typically added in small amounts to perform a specific function. Improve stability Minimize settling Reduce foaming Improving the flow out and wetting Increase pot life Increase UV resistance Increase or decrease gloss

7. S o l v ents Solvents are added to liquefy the binder. Once the coating is applied and cured, solvents serve no purpose. Solvency Power: The ability to dissolve the resin. Volatility: Governs the evaporation rate (the speed at which the solvent will leave the coating film during and after application).

8. Binder A coating typically gets its name from the binder used, such as: epoxy, polyurethane, alkyd, acrylic, etc. Good wetting and adhesion Resist transmission of water, oxygen, and other chemical species Resist chemical and physical change in the service environment Dry within an acceptable period Form a stable film that maintains its characteristic properties (strength, hardness, flexibility) Alkyd Resin

9. Modes of Protection Corrosion control by coatings can occur by one of only three processes: Barrier coatings Inhibitive coatings Sacrificial (cathodic protection)

10. Barrier Coatings The barrier coating obstructs the ingress of oxygen, water and soluble salts. St r u c tu r e NaCl and other ions O xygen Water Structure Surface (Steel) Protective Coating

11. Inhibitive Coatings Inhibitive coatings actively slow down the reaction occurring at the anode, cathode, or both: Must be in contact with the substrate. Actually passivate the metal surface. St r u c tu r e NaCl and other ions Oxygen W a t er Structure Surface (Steel) Protective Coating Inhibitive Primer Anodic and Cathodic reaction Inhibited

12. Sacrificial Coatings Sa c r ifici a l c o a tings us e a m e t al th a t is anodic to steel that corrodes preferentially. Sacrificial coatings: U s uall y c o n t ain zinc predominant pigment. du s t as the Must have a minimum loading of zinc dust to be effective. Holiday Local Cathode protection at holiday Structure Surface (Steel)

13. Adhesion Strong adhesion is the key to coating performance and long life. Adhesion can be: Chemical – Formed by a reaction between the coating and the substrate. Mechanical – Associated with surface roughness or anchor pattern. Polar – Most common for organic coatings. The resin acts as a weak magnet on the substrate. Combination of all three

14. Coating Types and Curing Mechanism

15. Curing Mechanism Curing is used to describe the way a coating transforms from a liquid to a solid state There are two broad classifications for curing mechanisms: Nonconvertible – Cure by evaporation of the solvent with no chemical change to the resin. Convertible – Undergo a chemical change during cure and cannot be returned to their original state.

16. Evaporation Cure Cure solely by solvent evaporation Can be re-dissolved in the solvent Examples include vinyl and chlorinated rubber

17. Coalescence Resin are dispersed in water. The water evaporates, the resin particles fuse (coalesce). Typically known as latexes or acrylic latexes.

18. Convertible Coatings Cure by one of several polymerization mechanisms Resins undergo a chemical change Not readily re-dissolved in the solvent Known as thermosetting materials Some examples of curing types are: Oxidation Co-reaction Hydrolysis Fusion

19. Oxidation Cure by reaction with atmospheric oxygen Unsuitable for immersion service Cannot withstand an alkaline environment, due to “saponification” Excessive film build may stop curing of the lower portion of the coating film Example: Alkyd

20. Co-Reaction Cure by polymerization reactions (cross-linking) between at least two chemical entities. Examples include: – Epoxies, Polyurethanes, Polyureas, Polyaspartics, Polysiloxanes and several others

21. Co-Reaction For solvent bone coatings, first step is solvent evaporation and second step is polymerization. Once mixed together, applicator has a short time to apply before it gels. The time period when the mixed material is usable is called pot life . Some co-reaction cured coatings also require a period of time after mixing but before application for the chemical reaction to start; this is referred to as the induction time .

22. Hydration Coatings require some amount of water to complete the cure. Examples include: Moisture cured polyurethane Solvent based inorganic zinc coating based on an ethyl silicate

23. F usion Forced heat curing May be single or two component materials Example is fusion-bonded epoxy (FBE)

24. Curing Mechanisms

25. Selection of Coating The selection of a coating is based on many factors that include: Service environment of the coating: Interior or exterior Immersion or atmospheric Chemical In-service temperature (plus typical range and upset conditions) Substrate being coated Size and configuration of item to be coated Surface preparation available and possible at job site

26. Selection of Coating Application temperature and humidity Life expectancy of both item being coated, and coating Ability of applicators Availability of application equipment Critical safety requirements, e.g., a nuclear power plant, buried pipeline, or commercial ship Budget

27. Generic Coating Types Acrylic Alkyds Chlorinated Rubber Epoxy Latex (Emulsions) Phenolic Polyaspartic Polyesters Polysiloxane Polyurethane Polyureas Silicones Vinyl Esters Vinyl Zinc (Inorganic) Zinc (Organic)

28. Acrylic Excellent UV and weathering resistance Can be applied as coalescence curing emulsions/ water dispersions Historically applied as decorative coatings rather than for corrosion resistance

29. Al k y ds Oxidative curing, referred to as “oil based paints” Single package material and broad range of colors Can be very slow curing products, limited thickness per coat Can be blended with other resins to improve properties e.g. With epoxies to make Epoxy Ester With silicones to make Silicon Alkyds Urethane alkyds by mixing with Isocyanates

30. Chlorinated Rubber Evaporation curing Contains a large amount of VOC Eliminated in most parts of the world Excellent resistance to water, sunlight, and many chemicals Should not be over coated with two component coatings

31. Ep o xy Two components consist of an epoxy resin (base) and a curing agent (converter) Can be solvent-based, water-based, or solvent-free Excellent adhesion, chemical resistance, water resistance, and wet adhesion Amine cured epoxies are especially sensitive to amine blush Exhibit chalking with atmospheric (UV) exposure

32. Latex (Emulsions) Resins normally thermoplastic resin types Coalescence curing

33. Phenolic Typically used where low pH environments and higher temperatures are factors Excellent resistance to acids Usually use for internal tank lining of corrosive material storage tanks Can be blended with epoxies to make Epoxy Phenolic

34. Polyaspartic Use to achieve low- or near-zero-VOC systems Pot lives from five minutes to several hours Film builds up to 380 μm (15 mils) DFT in a single pass Are aliphatic Polyureas

35. P o l y es t ers Have a short pot life Glass flake reinforced, high build coatings Excellent moisture resistance Exceptionally high abrasion resistance

36. Polysiloxane Used in services with abrasion, chemicals, extreme UV, and high temperature Three major categories : Inorganic Polysiloxane Epoxy-Polysiloxane Hybrids Acrylic-Polysiloxane Hybrids

37. Polyurethane Two major types Aliphatic More resistant to UV exposure, excellent gloss and color retention Aromatic Better chemical resistance in immersion but not UV resistant Main hazard is the isocyanate component Available with a variety of curing times

38. P o l yu r eas Very flexible materials Very short cure times Many require the use of an epoxy primer on steel

39. Silicones Formed by chemical modification of quartz, sand, or silicon Excellent high temperature and UV resistance Most require heat to cure but some cure at ambient temperatures Also used as foul-release coatings in the marine industry

40. Vinyl Esters Often referred to as linings Normally two-component coatings Excellent resistance to most of the corrosive chemicals even at higher temperatures Have rather short pot life Excellent abrasion resistance with glassflake added

41. V i n y l One of the earliest industrial coatings Were used on highway bridges and extensively in the marine industry Banned from use in most countries due to high VOC

42. Zinc (Inorganic) Widely used primer for steel structures Provides cathodic protection Very resistant to different chemicals and especially solvents Very high heat resistance with a max of 400°C (750°F) Disadvantages Can be difficult to apply Not for acidic environments

43. Zinc (Organic) Very different from inorganic zinc Organic resin with zinc filler Examples epoxy, polyurethane, etc. Limited cathodic protection factor

44. Environmental Testing

45. Environmental Effects Environmental, or ambient, conditions can greatly affect all phases of a coating operation.

46. Surface Temperatures Surface temperature is often different from air temperature. Application at incorrect temperatures can cause defects. Minimum and maximum application temperatures should be recognized. Substrate should be at least 3°C (5°F) above the determined dew point. Minimum Temperature Maximum Temperature

47. Relative Humidity Measure of the amount of moisture in the air compared to saturation level. May affect the coating if too high or too low. Too high may cause solvent entrapment.

48. Wind Speed Wind speed can affect the coating job by: Blowing abrasives Causing excessive overspray Accelerating solvent evaporation Contributing to the formation of dry spray. Dew Point The temperature at which moisture will begin to form on a steel surface.

49. Surface Temperature Instruments Typical instruments you may encounter include: Digital Infrared Thermometers Mechanical Surface Contact Thermometer Electronic Surface Contact Thermometer

50. Digital Infrared Thermometer Positives Are very simple: point, shoot, and read Deliver quick reading Negatives Steam, dust, smoke, and/or vapors can prevent accurate readings Cannot measure reflective surfaces accurately

51. Mechanical Surface Contact Thermometer Magnetic surface contact thermometers are one of the most common instruments Positives Simple and inexpensive Negatives Stabilization time can be minutes Easily lose their accuracy Heated Surface

52. Electronic Surface Contact Thermometer Magnetic surface contact thermometers are one of the most common instruments Positives Quick and accurate Negatives More expensive than other surface

53. Relative Humidity Instruments Electronic Hygrometers Sling Psychrometer Powered Air Flow Psychrometer

54. Electronic Hygrometer Read and calculate the: Temperature Relative humidity Dew point

55. Sling Psychrometer Sometimes called a whirling hygrometer Used to measure the ambient air temperature Dry-bulb temperature and wet-bulb temperature used to calculate the dew point and relative humidity

56. Powered Airflow Psychrometer Operates in a similar way to the sling psychrometer, but air is moved using a fan, rather than slinging the instrument.

57. Chart Recording Ambient Conditions Time RH% Dew Point Ambient Temperature Substrate Temperature Ok to work Yes/No

58. Wind Speed Wind speed can be a safety issue when work is being performed at heights. Monitoring wind direction can prevent damage to property from overspray.

59. Product Technical Data Sheet And Material Safety Data Sheet

60. Product Technical Data Sheet Coatings Product Data Sheets (Paint Specs) provide users with valuable information on the application aspect of a particular product. Communicate technical facts related to the specific material and its application properties. Common to find a small section related to safety.

61. Product Technical Data Sheet Information of critical importance to the project success: Surface preparation Storage Mixing and thinning Application procedure DFT requirements One should always consult Product Technical Data Sheets. It may come across discrepancies between the project specification and the product data sheet. The holder of the specification (owner) must be contacted for clarification

62. Product Technical Data Sheet A thorough understanding of the Product Technical Data Sheet cannot be over emphasized!

63. Material Safety Data Sheet Contains data regarding the properties of a particular substance. Provides workers and emergency personnel with critical information on composition, handling, or working with that substance in a safe manner. Includes information such as melting point, boiling point, flash point, toxicity, health effects, first aid, reactivity, storage, disposal, protective equipment, and spill handling procedures. Provides information regarding the safety issues associated with any hazardous (or potentially hazardous) material. Provides instructions for the correct action to take in the event of a spill, explosion, fire, or hazardous exposure.

64. Material Safety Data Sheet When coating materials are being transported, there is a constant risk of spillage or exposure of others to potentially toxic chemicals. In some countries the law requires that MSDS be carried by transporters whenever industrial quantities of coatings are moved by road, rail, or air. Inform emergency services of potential hazards in the event of a spill, fire, or other hazard. Chemicals commonly used and judged to be hazardous are listed by various industry or national authorities.